What does the website DO? Why do people log in to it? What does it do for them?
I'd be incredibly wary of any software that "scans" my computer and takes screen shots while communicating secure credentials to a 3rd party website. I'll assume you have all of those problems worked out.
I would probably solve this by exposing a secure web service. Going from not knowing what version of
VB you're using to secure web service architecture is going to be more than a little blip, fair warning...
The basic idea is you have a web service that communicates xml back and forth between your website and whoever wants to talk to it. In this case "whoever wants to talk to it" would be your desktop application. The web service listens for incoming xml, parses it out and decides what to do. The service could be a PHP script, .NET service, etc. Just so long as you can send it specifically formatted xml that tells it what to do, and receive formatted xml with the data you want back.
If, for instance, the web service is an identity management service, it might receive a request to validate whether a username/password is correct. The service will receive the username/password as xml, decide it has to go to the database to validate the credentials, and then emit xml back to the caller. The caller than has to parse the xml it received from the web service to determine what the answer was.
That was a simplified example, but hopefully enough to get the picture.
I strongly, STRONGLY recommend going through a lot of basic tutorials before trying to hack this together. Once you have the basic concepts down,
VB.NET can do a lot of the heavy lifting for you on the desktop app side. It's important to understand what it's doing for you before deploying something like this to the internet though. I'm not very familiar with PHP so I don't know how painful it will be to write your service there. That's why asked how far along you are with your website. .NET has facilities to write the irritating parts of the service for you, complete with out-of-the-box authorization and authentication providers.
I'll be blunt. The fact that you copy and pasted VB6 code into a .NET application and passed unsecured credentials via GET parameters to an unsecured, unsanitized PHP script does not bode well...